1. Field of the Invention
The present invention relates to a noninvasive device for photoelectrically measuring a property of arterial blood, such as an oximeter or a densitometer for measurement of a pigment in blood.
2. Description of the Prior Art
The behavior of light in materials has been a topic of study in various theoretical works such as disclosed in "New Contributions to the Optics of Intensely Light-Scattering Materials. Part 1," by Paul Kubelka, Journal of the Optical Society of America, Volume 38, No. 5, May 1948; "Optical Transmission and Reflection by Blood," by R. J. Zdrojkowski and N. R. Pisharoty, IEEE Transactions on Bio-Medical Engineering, Vol. BME-17, No. 2, April, 1970, and "Optical Diffusion in Blood," by Curtis C. Johnson, IEEE Transactions on Bio-Medical Engineering, Vol. BME-17, No. 2, April, 1970.
On the other hand, various practical devices or methods for measuring blood property have been disclosed in the patent literature such as U.S. Pat. Nos. 3,368,640, 3,998,550 and 4,086,915, and Japanese Patent Publication No. 53-26437.
In the specific field of the medical-optical art noninvasive measurements relating to the amount of a pigment in the blood, such as hemoglobin, hemoglobin oxide, bilirubin or an artificially injected pigment, have generally taken the following form. The oximeter usually comprises means for providing a source light; means for photoelectrically measuring the intensity of the source light after contact with a living tissue containing the arterial blood at a first wavelength, at which the light absorption coefficients for hemoglobin and hemoglobin oxide are equal, and a second wavelength, at which the two light absorption coefficients greatly differ from each other, to produce a pair of electric signals, respectively, the signals each include an alternating-current (AC) component and a direct-current (DC) component; means for calculating information representative of the amplitude of the alternating-current component relative to the direct-current component with respect to the first and second wavelengths to produce a first and second calculated output, respectively; means for presenting a final output indicative of the oxygen saturation, and means for relating the final output with the first and second calculated outputs so that the final output is a linear function of a ratio between the first and second calculating outputs.
However, clinical experiences have recently reported that an oximeter of the above type was apt to show some aberration or error of measurement in the relatively lower oxygen saturation range although the measurements are quite accurate in the higher oxygen saturation range. Thus there is still a need in the prior art to provide improved electro-optical measuring devices for medical use.